Thermal ink-jet printers offer a low cost, high quality, and comparatively noise-free option to other types of printers commonly used with computers. Such printers employ a resistor element in a chamber provided with an egress for ink to enter from a plenum. The plenum is connected to a reservoir for storing the ink. A plurality of such resistor elements are arranged in a particular pattern, called a primitive, in a printhead. Each resistor element is associated with a nozzle in a nozzle plate, through which ink is expelled toward a print medium. The entire assembly of printhead and reservoir comprise an ink-jet pen.
In operation, each resistor element is connected via a conductive trace to a microprocessor, where current-carrying signals cause one or more selected resistor elements to heat up (resistive heating). The heating creates a bubble of ink in the chamber, which is expelled through the nozzle toward the print medium. In this way, firing of a plurality of such resistor elements in a particular order in a given primitive forms alphanumeric characters, performs area-fill, and provides other print capabilities on the medium.
Many inks that are described for use in ink-jet printing are usually associated with non-thermal ink-jet printing. An example of such non-thermal ink-jet printing is piezoelectric ink-jet printing, which employs a piezoelectric element to expel droplets of ink to the medium. Inks suitably employed in such non-thermal applications often cannot be used in thermal ink-jet printing, due to the effect of resistive heating on the ink composition.
Cockle refers to the swelling and deformation of paper while wet with ink, and also when dried. Wet cockle is defined as paper that is cockling immediately after printing; dry cockle, also called curl, is defined when the paper has dried. The main emphasis of this invention is on minimizing wet cockle.
The printhead-to-paper spacing is a specific, definite value (assuming flat, non-cockled paper), and if wet paper (cockled) swells high enough, a crash between the printhead and paper will occur and result in deleterious effects during printing, such as print quality degradation and possible printhead malfunctions. As such, minimizing cockle is of primary importance for ink-jet printing.
Without subscribing to any particular theory, it is believed that the reason paper cockles when wet is due to the absorption of water by paper fibers and breaking of interfiber bonds. This results in expansion of the paper and a series of waves that appear across the printing zone. If the amplitude of the waves is greater than the head-to-paper distance, interference will occur between the two. One approach to solving the cockle problem would be to find an "anti-cockle" additive that could be placed in the ink at low enough concentrations to minimize wet cockle without adversely affecting the performance of the pen or output. Such anti-cockle additives are disclosed and claimed in U.S. Pat. No. 5,207,824, issued May 4, 1993, and assigned to the same assignee as the present application. While this approach is certainly suitable, efforts have continued to find other solutions to the cockle problem.
A need remains for a reformulation of certain thermal ink-jet inks to avoid paper cockle.